BMW 335d Glow Plug Control Module Replacement Cost

Diesel engines are a type of internal combustion engines (ICE) which operate slightly different than the normal gasoline engines that most are accustomed to. Instead of sing a spark to ignite fuel, like gasoline ICEs, they use high pressures and heat to ignite fuel. However, when a diesel engine is first started, there is no heat present, so diesel fuel has no way to ignite. Enter to glow plug. The glow plug gets its name because it becomes so electrically heated that it glows red. This is only needed when the engine is cold, and when the engine reaches a certain temperature, the glow plug control module turns them off.

When the ignition switch is turned from off to start, the glow plug control module does a few things. It prevents the engine from cranking, turns the glow plugs on, lights up a warning lamp that instructs the driver to wait, and finally, shuts down the warning and allows the driver to start the engine, or starts the engine itself. This system prevents excessive emissions, carbon build up, long cranking time which drains batteries, and misfires when starting the diesel engine. Also, this control module can notify the driver that a glow plug has failed.

A faulty glow plug control module will generally lead to a diesel engine that fails to start, fails to start properly, or runs poorly once started. The warning for the driver to wait may not come on, or may stay on indefinitely, which would either prevent the engine from cranking or allow the engine to crank at any time. Also, the glow plugs could be heated to the incorrect temperature, not heated at all, or never turned off. This could cause pre-ignition issues, misfires at startup or failure to start. In general, the only time a glow plug control module should affect engine performance is before, during, or in the few minutes following a cold-start.

Most diesel powered vehicles have an optional block warmer. This engine block warmer, if equipped can eliminate the need for glow plugs since the engine should be warm enough to fire on its own. This will likely not work in very cold weather. Also, experienced diesel owners may also understand how to jump the glow plugs, and this will allow them to manually warm the engine. However, these practices are only temporary relief from failure of the glow plug control module, and only if the module will not warm the glow plugs. In all other situations, the glow plug control module will require immediate repair to operate or prevent further damage.

Glow plug control module failure, like other electronic components, is random, and predictability is guesswork at best. Since this module is only used for a few minutes each time the vehicle is started, it is not under high stress or constant load, and, theoretically, should last the lifetime of the vehicle. The caveat to that consideration is that diesel powered vehicles tend to last about 2-3 times longer than their gasoline powered counterparts, so the possibility of replacing many components during the lifetime of the vehicle is much higher.

When diagnosing the glow plug control module, the technician will simply compare actual conditions to electronic conditions. In other words, If the technician notes that the glow plugs are inactive, yet the control module reports they are ready to start, a discrepancy is noted. Discrepancies like this are commonly used as clues which map to the issue. If the technician finds that the issue originates with the glow plug control module, whether by physical or scan testing, the module will be verified defective and replaced.

Like any other control module, the location of the glow plug control module will be identified, and the technician will then begin removing all obstructing components needed to access and remove the control module. Once accessed and removed, the connectors are cleaned and inspected, and the new module will be installed. After all components are reassembled on the vehicle, the technician will program the control module, if needed, and test the operation of the system to ensure the customer complaint has been corrected.

When dealing with electronic systems, especially if the issue pertains to the control module, we recommend thorough diagnostics before replacing parts. In effort to correct a no-start condition for a diesel engine, it would be very easy to spend hundreds of dollars replacing glow plugs, the glow plug harness, grounding straps, relays, and batteries before realizing the issue is more advanced. In the case of a failure relating to the glow plug control module, starting, or rough running on start-up, diagnostics is a must.

Diesel engines are a very industrial style of engine, and even modernized versions of diesel engines are more dangerous to service than gasoline engines. The parts and systems are hardly comparable, and there is great danger of heat exposure and electric shock when dealing with the glow plugs. Great care should be taken to understand the engine control systems of a diesel engine before attempting to service them.

Except for the very advanced DIY mechanic with a strong understanding of computerized circuit diagnostics and testing, this repair should be left to a professional. Again, the glow plug circuit and glow plugs can cause serious harm if interacted with improperly, and diagnostics of electronic systems are difficult for the untrained person to perform correctly. Unless all safety and diagnostic protocol is understood and followed properly, this should be left to a professional.